The measurement of the bath temperature of molten metal, such as molten steel, in a basic oxygen furnace is very important to the refining and subsequent processing of the liquid steel produced in the furnace. One well-used method of measuring the molten steel temperature is to temporarily interrupt the refining process, tilt the furnace to a generally horizontal position, and manually insert a consumable probe containing a temperature sensor or other sensors to a particular depth in the liquid steel. While this method has been effective in determining the molten steel temperature and other parameters, it is time consuming and quite disruptive of the steel-making process.
During the mid-1960's, so-called "throw-in" thermocouple sensor devices were introduced to permit the measurement of the temperature of the molten steel while avoiding the costly, time-consuming procedure involved in the tilting and manual measurement process. Typical throw-in sensor devices employed at that time are described in U.S. Pat. Nos. 3,374,122 and 3,497,398. The sensor devices shown in these patents employ a standard or typical thermocouple-type sensor attached to a paper or cardboard support tube and a separate weight element which surrounds at least a portion of the support tube for the purpose of causing the sensor device to sink into the molten steel. The furnace would remain in an upright position and the sensor device would be dropped approximately 60 to 70 feet into the molten steel in the furnace. A leadwire of a suitable length connected the thermocouple sensor to instrumentation located outside of the furnace for interpreting the sensed temperature of the molten steel. Such sensor devices were deficient due to their tendency to float at the slag/metal interface which often resulted in inaccurate temperature measurements. The flotation problem was primarily a result of the sensor devices having a net density which, despite the additional weight element, was less than the density of the liquid steel. Such sensors also had a high center of gravity which resulted in inaccurate measurements.
An alternate method of making temperature measurements in a basic oxygen furnace employed a motorized lance or probe with multi-purpose temperature and/or other sensors which also did not require the tilting of the furnace or the interruption of the refining process. Such motorized systems, while providing generally good temperature measurement results, required multi-million dollar expenditures for system installation and were also demonstrated to be costly to operate and maintain.
More recent developments in basic oxygen furnace throw-in sensor devices are disclosed in U.S. Pat. Nos. 4,881,824 and 5,275,488. U.S. Pat. No. 4,881,824 discloses an immersible probe having a counter-weight and float which is employed to maintain a temperature sensor at a prescribed depth for the proper measurement of the molten steel temperature. The described probe has a net density which is less than that of liquid steel and has a high center of gravity, resulting in the probe maintaining a generally vertical orientation in the molten steel only as long as the slag layer above the steel is of a sufficient minimum thickness. U.S. Pat. No. 5,275,488 discloses a probe having a net density which is greater than the density of the molten steel. However, this patent does not address additional factors such as entrapped gas buoyancy and high center of gravity, both of which detrimentally affect the effectiveness of the temperature measurement.
A probe having a density greater than that of molten steel will not necessarily sink into the molten steel, specifically a high oxygen, low carbon steel typically present in a basic oxygen furnace. Gas evolution from the carbon-oxygen reaction at the surface of such a probe results when the relatively cold sensor head of the probe contacts the highly oxygenated steel in the steel bath. The gas evolution at the sensor head/liquid steel interface results in a flotation force being applied to the sensor head which pushes the probe upwardly away from the area at which the temperature measurement should be made. The probes disclosed in both patents include a rigid metallic tube over the leadwire at the sensor head end to prolong the life of the leadwire in the molten steel bath. Although protecting the leadwire from the molten metal, the rigid metallic tube creates a higher center of gravity for the probe which results in vertical instability of the probe when immersed in the liquid steel. The shape of both of the probe sensor heads is not particularly conducive to deep probe penetration into the molten metal. In addition, the use of metal support legs, as shown in U.S. Pat. No. 4,881,824 in the vicinity of the sensor element results in temperature measurement errors due to thermal gradients produced by liquid steel solidification on the support legs as well as the previously described gas evolution which occurs during the initial cold probe immersion into the molten steel bath.
Based upon the foregoing, it has been determined that the forces tending to minimize penetration of the sensor head of a probe into the molten steel are those resulting from viscous drag of the gas atmosphere within the furnace, slag and liquid steel, the retarding force of the trailing probe leadwire, the net density of the probe as compared to the density of the liquid steel, the effective density decrease as a result of slag adhering to the sensor during insertion into the molten steel, and gas evolution at the sensor head due to metal solidification. The molten steel circulation in the furnace also aids or retards sensor head penetration into the molten steel. If all of these factors result in a net downward force, the sensor head continues to sink until the sensor leadwire is taut or the probe contacts the bottom of the furnace. If these forces result in a net upward force, the probe rises to the slag metal interface or into the slag.
The present invention comprises a drop-in consumable immersion probe designed to economically increase probe penetration into the liquid steel while minimizing retarding and buoyancy forces. The penetration force of the present probe is increased by increasing the effective probe density using steel for the sensor head and minimizing internal cavities by using a miniature thermocouple element and filling all remaining voids in the sensor head with a dense particulate material. The retarding forces are further minimized by providing the probe with a projectile-like shape which is conducive to deep penetration of the probe into the molten steel. The projectile-like shape minimizes gas entrapment as well as slag and molten steel drag on the probe during immersion. The slag cap of the present probe as well as the steel measurement head are preferably provided with an ablative coating to further retard slag adherence. The conical shape of the probe measurement head also minimizes thermal gradients in the area of the temperature sensing element resulting in a more representative temperature measurement of the molten steel. Finally, a heat resistant oversleeve is provided around at least the portion of the sensor leadwire exposed to the molten steel for extending the life of the leadwire when the probe is immersed into the molten steel.